From: John Feminella <johnf@...>
Date: 2011-05-08T07:55:54+09:00
Subject: Re: Help with while condition OR condition

Oops, small typo:

    while command != "exit" || command != "quit" ...

should be:

    while command != "exit" && command != "quit" ...
--
John Feminella
Principal Consultant, BitsBuilder
LI: http://www.linkedin.com/in/johnxf
SO: http://stackoverflow.com/users/75170/



On Sat, May 7, 2011 at 18:53, John Feminella <johnf@bitsbuilder.com> wrote:
> This line doesn't behave in the way I think you expect:
>
>> while input.chomp.downcase != (EXIT || QUIT) #only works for exit
>
> This says, "check if the downcased input is not equal to the value of
> the expression `EXIT || QUIT`". What is the value of that expression?
> In this case, it will resolve to EXIT, since the string "exit" is not
> false or nil and is thus true. The value of QUIT is never evaluated.
> So, if the input is not equal to exit, the while loop continues.
>
> Why is that? In Ruby, all expressions have both a "value" and a
> "truthiness". An expression is "falsy" if it evaluates to either
> `false` or `nil`; otherwise it is `truthy`. In the case of an
> expression like `foo || bar`, the truth table would look like this:
>
> * foo is truthy, bar is truthy: result is `foo` and truthy
> * foo is truthy, bar is falsy: result is `foo` and truthy
> * foo is falsy, bar is truthy: result is `bar` and truthy
> * foo is falsy, bar is falsy: result is `bar` and falsy
>
> So you can see that in your case, foo is EXIT and bar is QUIT, both of
> which are truthy values; thus the expression is "exit". To get what
> you what want, try something like this:
>
>    command = input.chomp.downcase
>    while command != "exit" || command != "quit" ...
>
> Or, more succinctly:
>
>    case command
>    when "exit", "quit"
>      puts "exiting!"; ...
>    else
>      # do stuff
>    end
>
> ~ jf
> --
> John Feminella
> Principal Consultant, BitsBuilder
> LI: http://www.linkedin.com/in/johnxf
> SO: http://stackoverflow.com/users/75170/
>
>
>
> On Sat, May 7, 2011 at 18:37, Bill W. <sirwillard42@gmail.com> wrote:
>> Hi everyone,
>>
>> This is my first post, so I hope I don't sound too inexperienced..
>>
>> I'm trying to teach myself Ruby, and have run into an issue with a while
>> statement that will break if an input is "exit" or "quit".
>> As of right now, it works if exit is input, but not quit
>>
>> I know I am completely misusing the entire thing, but here is what I
>> came up with:
>>
>> EXIT = "exit"  #need constants since Ruby gets pissed at string literals
>> QUIT = "quit"  #in a comparison
>>
>> print "Input: "
>> input = gets
>> while input.chomp.downcase != (EXIT || QUIT) #only works for exit
>>
>>  #Do something
>>
>> print "Input: "  #pick up the next input and check it
>> input = gets
>> end
>>
>> I know that Ruby has a lot of shortcuts, but if you post any please
>> explain how they work (or provide a link to a good explanation.
>>
>> Thanks!
>>
>> --
>> Posted via http://www.ruby-forum.com/.
>>
>>
>